1. Field of the Invention
This invention relates generally to automatic test equipment for electronics, and, more particularly, to techniques for generating periodic signals for testing electronic devices.
2. Description of the Related Art
Electronics manufacturers commonly use automatic test equipment (ATE) for testing semiconductor components and electronic assemblies. ATE reduces costs to manufacturers by allowing products to be tested early in the manufacturing process. Early testing allows defective units to be identified and discarded before substantial additional costs are incurred. In addition, ATE allows manufacturers to grade different units according to their tested levels of performance. Better performing units can generally be sold at higher prices.
One of the basic functions of ATE is to generate signals of predetermined frequency. These signals may include, for example, digital clocks, analog waveforms, and RF waveforms. Often, particular testing scenarios require a test system to produce multiple signals of different frequency. Commonly, frequency and phase differences between different signals must be precisely controlled. Phase-locked loops are commonly used in ATE systems to produce signals with precisely controlled frequency and phase.
FIG. 1 shows a block diagram of a conventional phase-locked loop (PLL) 100. The PLL 100 receives an input signal, FIN, and generates an output signal, FOUT. The PLL 100 includes a phase detector 110, a loop filter 112, and a voltage-controlled oscillator (VCO) 114. It also includes an output frequency divider 118 and a feedback frequency divider 116. The input signal, FIN, may be supplied by any suitable source, such as a crystal oscillator.
The conventional PLL 100 is a closed loop feedback system that operates essentially as follows. The phase detector 110 compares the input signal FIN to the feedback signal FFB to generate an error signal, which varies in relation to the difference in phase between FIN and FFB. The loop filter 112 smoothes the error signal and generally helps to stabilize the feedback loop. The VCO 114 converts the filter's output signal into an oscillatory signal, FVCO, which has a frequency that varies in relation to the filter's output signal. The feedback divider 116 (generally a counter) divides the frequency of FVCO by an integer, M, to produce the feedback signal, FFB. Outside the loop, the output divider 118 divides the frequency of FVCO by an integer, N, to produce FOUT. As the feedback tends to drive the difference between FIN and FFB to zero, it consequently drives the frequency of FVCO to a value equal to the frequency of FIN*M, and therefore tends to drive the frequency of the output signal FOUT to a value equal to the frequency of FIN*M/N.
The conventional PLL 100 provides many benefits. For example, output frequency FOUT can be varied, through appropriate selection of N and M, over a wide range of values. In addition, phase noise in the PLL can generally be reduced by setting the bandwidth of the loop filter 112 to arbitrarily low values.
Nevertheless, we have recognized certain shortcomings in the PLL 100, which limits its usefulness in many ATE applications. High frequency applications, such as RF signal generation, require high frequency VCOs. The speed of the VCOs in these applications often greatly exceeds the speed of the phase detectors. This problem is conventionally addressed by making the value of M in the feedback divider 116 very large.
Making the value of M large involves certain drawbacks, however. For instance, the larger the value of M, the greater the reduction in the open-loop gain of the PLL 100. As is known, reducing open-loop gain increases loop tracking errors. It also impairs the ability of the loop to reject noise. To illustrate this effect, consider that the feedback divider 116 not only divides the frequency of FVCO by M, but it also divides any variations (i.e., phase noise or, equivalently, timing jitter) by the same value of M. Sensitivity is therefore reduced.
The frequency divider 116 also adds noise directly. Frequency dividers are commonly implemented as counters, which are known to create spurious noise at their outputs. Although this noise can be attenuated by the loop filter 112, attenuation cannot generally be achieved without setting the bandwidth of the loop filter to a much lower frequency than the offending noise components of the divider 116. Reducing bandwidth to this degree, however, has the effect of reducing programming speed of the PLL 100, which can negatively impact ATE system performance and throughput.
What is desired is a phase-locking circuit that can produce high frequency signals with low phase noise, without sacrificing programming speed.